1. Field of the Invention
The present invention relates to an ink jet head driving method and driving apparatus for changing the capacity of a pressure chamber in which ink has been filled by a piezoelectric element in response to a print signal, and then, ejecting an ink droplet from a nozzle which communicates with the pressure chamber by the resulting pressure change, thereby printing a character or an image and the like on a printing medium.
2. Description of the Related Art
A description will be given with a conventional print head with reference to FIG. 13. In FIG. 13, reference numeral 1 denotes an ink jet print head. This ink jet print head 1 is composed of: a plurality of pressure generating chambers in which ink is filled; a nozzle plate 11 provided at one end of each of these pressure generating chambers 17; a nozzle 15 for ejecting an ink droplet 19 formed in correspondence with each of the pressure generating chambers 17 on this nozzle plate 11; a piezoelectric actuator 14 provided in correspondence with each of the pressure generating chambers 17 to apply vibration to the pressure generating chambers 17 via a vibration plate 13, and then, eject ink from the nozzle 15 by a capacity change inside of the pressure generating chambers 17 due to the applying of this vibration; and an ink chamber 18 or the like provided in communication with each of the pressure generating chambers 17, the ink chamber being adopted to supply the ink to the pressure generating chamber 17 via an ink supply passage 16 from an ink tank not shown. With such a construction, when the piezoelectric actuator 14 is driven, a pressure vibration is applied to the pressure generating chamber 17, the capacity inside of the pressure generating chamber 17 is changed by this pressure vibration, and the ink droplet 19 is ejected from the nozzle 15. This ink droplet 19 is deposited onto a printing medium such as printing sheet of paper, and a dot is formed on the printing medium. By continuous forming of such dots, a predetermined character or image and the like based on image data is printed.
In general, in an ink jet printer, in the case where high quality printing is carried out, there is used an area gradation system such as a dither system, for forming one pixel by producing a matrix with a plurality of dots without changing the size of an ink droplet, and expressing gradation based on a difference in the number of dots in pixel. In this case, resolution must be sacrificed in order to allocate a certain number of gradations. In addition, there is provided a density gradation system for changing the density of one dot by varying the size of an ink droplet. In this case, although resolution is not sacrificed, there is a problem that a technique for controlling the size of an ink droplet is difficult.
Further, there is a so called multi-drop driving system for carrying out density gradation by varying the number of ink droplets to be printed with respect to one dot without changing the size of an ink droplet. In this case, resolution is not sacrificed, and there is no need to control the size of an ink droplet, thus making it possible to comparatively easily carry out this driving system.
A method for driving an ink jet head in a multi-drop system is also known (refer to Jpn. Pat. No. 2931817). Further, an ink jet type printing apparatus is known as reducing a cycle of a drive signal so as to speed up printing (refer to Jpn. Pat. Appln. KOKAI Publication No. 2001-146003). Furthermore, an ink jet printing apparatus for, when a repetition time for ejecting ink droplets variously changes, efficiently ejecting a predetermined amount of ink from an ejecting port is also known (refer to Jpn. Pat. Appln. KOKAI Publication No. 2000-177127).
In this multi-drop driving system, in the case where a plurality of ink droplets are continuously ejected, an ejection speed of second and subsequent droplets can be increased more significantly than that in a first ink droplet by using residual pressure vibration of the droplets just ejected before.
On the other hand, in general, the first ink droplet becomes lower in ejection speed than the second and subsequent ink droplets because a pressure vibration is applied in a state in which meniscus is stationary. Thus, there is a problem that ejection becomes unstable or print quality is degraded because of a small amount of ejection.
In order to avoid such a problem, there is an option for increasing an applied voltage, and then, increasing a pressure vibration entirely applied to a pressure chamber, thereby increasing a first-drop ejection speed. However, there is a problem that power consumption is increased, and a heating rate is increased by increasing a voltage. In addition, there is a problem that ejection becomes unstable because the ejection speed of the second and subsequent droplets becomes too high or print quality is degraded due to displacement in ink deposition between gradations, resulting from the increased difference in ejection speed of each droplet.
In addition, another method for avoiding a problem that an amount of ejection is small and print quality is degraded includes increasing a first-drop ejection speed by applying a fine pressure vibration to an extent that a ink droplet is not ejected before a first-drop drive pulse (hereinafter, such a drive pulse is referred to as a boost pulse). This boost pulse is redundantly applied, whereby a time of an entire drive cycle is extended, and therefore, such an extended time is disadvantageous for high speed printing.